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Borrowings - Narrative (Details) - USD ($) |  |  |  |  |  |  | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 14, 2024 | Jun. 28, 2024 | May 15, 2024 | Apr. 11, 2024 | Apr. 09, 2024 | Mar. 13, 2024 | Sep. 30, 2024 | Dec. 31, 2023 | Jun. 14, 2024 | May 07, 2024
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Short-term borrowings |  |  |  |  |  |  | $ 0 | $ 0 |  |
Estimated weighted average life of student loans |  |  |  |  |  |  | 5 years 4 months 24 days | 5 years |  |
Vertical risk retention interest |  |  |  |  |  |  | 5.00% |  |  |
Uncommitted federal funds |  |  |  |  |  |  | $ 125,000,000 |  |  |
Lendable value of collateral |  |  |  |  |  |  | 2,300,000,000 | $ 1,600,000,000 |  |
Secured Borrowings | Line of Credit |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Secured borrowing facility maximum borrowing capacity |  |  |  |  |  |  |  |  | $ 2,000,000,000 | $ 2,000,000,000
Line of credit facility, interest holding percentage |  |  |  |  |  |  |  |  | 100.00% | 100.00%
Short-term borrowings |  |  |  |  |  |  | 0 | $ 0 |  |
2024-C |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing |  |  | $ 668,000,000 |  |  |  |  |  |  |
Loans sold to third parties |  |  | $ 668,000,000 |  |  |  |  |  |  |
Ownership interest percentage on asset-backed financing |  |  | 100.00% |  |  |  |  |  |  |
Unsecured debt issued |  |  | $ 668,000,000 |  |  |  |  |  |  |
Estimated weighted average life of student loans |  |  | 5 years 4 months 9 days |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 1.19% |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 716,000,000 |  |  |
Principal amount of collateral |  |  |  |  |  |  | 656,000,000 |  |  |
Capitalized interest |  |  |  |  |  |  | 60,000,000 |  |  |
2024-E |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing | $ 868,000,000 |  |  |  |  |  |  |  |  |
Loans sold to third parties | $ 868,000,000 |  |  |  |  |  |  |  |  |
Ownership interest percentage on asset-backed financing | 100.00% |  |  |  |  |  |  |  |  |
Unsecured debt issued | $ 868,000,000 |  |  |  |  |  |  |  |  |
Estimated weighted average life of student loans | 5 years 2 months 1 day |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 1.42% |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 933,000,000 |  |  |
Principal amount of collateral |  |  |  |  |  |  | 855,000,000 |  |  |
Capitalized interest |  |  |  |  |  |  | $ 78,000,000 |  |  |
2023-A |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Vertical risk retention interest |  |  |  |  |  |  | 5.00% |  |  |
2023-A | Private Education Loans |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loans sold |  |  |  |  |  | $ 2,000,000,000 |  |  |  |
Additional proceeds from sale |  |  |  |  |  | 105,000,000 |  |  |  |
Gain on sale |  |  |  |  |  | $ 7,000,000 |  |  |  |
2024-R1 |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Vertical risk retention interest |  |  |  |  | 5.00% |  |  |  |  |
2024-R1 | Private Education Loans |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loans sold |  |  |  |  | $ 69,000,000 |  |  |  |  |
2024-B |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Vertical risk retention interest |  |  |  |  |  |  | 5.00% |  |  |
2024-B | Private Education Loans |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loans sold |  |  |  | $ 191,000,000 |  |  |  |  |  |
Additional proceeds from sale |  |  |  | 10,000,000 |  |  |  |  |  |
Gain on sale |  |  |  | $ 1,000,000 |  |  |  |  |  |
2024-D |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Vertical risk retention interest |  |  |  |  |  |  | 5.00% |  |  |
2024-D | Private Education Loans |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |
Private education loans sold |  | $ 1,500,000,000 |  |  |  |  |  |  |  |
Additional proceeds from sale |  | 79,000,000 |  |  |  |  |  |  |  |
Gain on sale |  | $ 6,000,000 |  |  |  |  |  |  |  |

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